It is well known that if two surfaces of insulating materials are rubbed together and then separated, an electrostatic charge will build up between the two surfaces. In recent years, this problem in computer room flooring and desk areas has been troublesome, since the discharge of built-up static can result in tape or disc erasures and interference with sensitive equipment. Such charged surfaces in hospital surgical, and other areas where certain anesthetic gases can form violently explosive mixtures with air, has caused even greater concern that the chances of explosions caused by sparks or electrical discharges be minimized. In all of these cases, the static build-up can be caused by walking on flooring, moving electronic components or other equipment from place to place, and even utilizing the keyboard on a computer terminal. Such static build-up can also occur over a period of time in the wearing apparel of workers.
The need for spark-proof flooring was recognized many years ago by Donelson et al., in U.S. Pat. No. 2,351,022. There, calcined magnesite, MgO, was mixed with from about 40 wt. % to 60 wt. % of finely divided coke particles, having from 1/8 inch screen size to fine dust, and liquid magnesium chloride, to provide a spreadable floor composition which could be troweled over a concrete, steel, or wood sub-floor. Such flooring was not very resilient, however, and caused fatigue to those who had to stand or walk on it all day.
More recently, Charlton et al., in U.S. Pat. No. 3,040,210, taught a much more resilient, decorative, carbon containing, linoleum floor sheeting, laminated to a conductive base. The linoleum surface sheeting contains from 1 wt. % to 14 wt. % conductive carbon, homogeneously mixed with other conductive materials, linoleum binder, which contains oxidized drying oils such as linseed oil with up to 35 wt. % resin such as rosin ester gum or phenol-formaldehyde, and sufficient coloring pigments to provide an attractive appearance. The conductive backing must contain from 10 wt. % to 35 wt. % conductive carbon, and can be bonded to fabric for added strength, where the fabric itself can be made conductive by initially dipping it in a dispersion of conductive carbon. This provides a static resistant flooring having a controlled electrical resistance, which will wear evenly, can be applied in long sections minimizing seams, and which is resilient enough to help reduce fatigue for people that must stand or walk on the flooring for long periods of time.
Berbeco, in U.S. Pat. No. 4,301,040, taught static-free mats containing a standard, non-conductive decorative laminate, such as a 0.16 cm. (1/16 in.) thick melamine-formaldehyde laminate, or a rubber, nylon, polycarbonate, polyethylene or polypropylene, non-conductive sheet, as a top surface, adhesive bonded to, or coated with, either an electrically conductive solid or an open cell foam bottom backing layer. The bottom layer includes a polymeric material or a foam and an antistatic amount, generally about 2 wt. % to 40 wt. % of conductive particulate material, such as metal particles, aluminum salts such as aluminum silicate, graphite fibers, and preferably carbon black particles. Useful polymeric materials include butadiene-styrene resin and the like, and useful foams include polyurethane foams, polyester foams and the like. When a foam is used as the bottom layer, a flexible cushion mat results.
Standard decorative laminates are non-conductive through their cross-section, and are described, for example, by McCaskey, Jr. et al., in U.S. Pat. No. 4,061,823. They are popular as surfacing material for counter and furniture tops. Because, in many cases, they must be machined, fillers other than coloring pigments are usually avoided. Such laminates generally contain 2 to 6 fibrous, Kraft paper sheets, impreganted with phenol-aldehyde resin, as a core for 1 high quality, fibrous, alpha-cellulose decorative print sheet, having a pattern or plain color, impregnated with melamine-aldehyde resin, and 1 top, high quality, fibrous, alpha-cellulose overlay protective sheet, also impregnated with melamine-aldehyde resin. Any pigmentation fillers would only be present in the decorative print sheet.
The Donelson et al. composition is applicable to dense flooring and requires large amounts of carbon material. The Charlton et al. material also requires the use of large amounts of relatively expensive carbon, and requires a complicated manufacturing process. The Berbeco material requires a non-conductive surface, through which the backing would have to draw static charges. Of course, standard decorative laminates are usually non-conductive. What is needed is a surfacing material useful for flooring and desk or counter tops, having outstanding antistatic properties, good wear properties, and an attractive appearance, and which is also inexpensive, easy to manufacture, and thin enough to allow ease of installation.